1. Field of the Invention
The invention relates to the field of optical recording of information on disk or tape and reading the information; and is most closely related to optical lens systems for focusing laser beams for such optical storage and/or retrieval.
2. Description
The invention relates to an optical device for optically scanning an information plane, comprising a radiation source for supplying a scanning beam, and an objective system for focusing the scanning beam to a scanning spot on the information plane.
The invention also relates to an apparatus for reading and/or writing an optical record carrier provided with such a scanning device.
The information plane may be the plane of an optical record carrier in the form of a disc or a tape in which the optically readable information is present or may be stored. Such a record carrier is, for example, the known compact disc (CD) for audio or CD-ROM for data or a record carrier derived therefrom, such as the DVD. The information plane may also be the surface of another object which can be examined, for example, with a scanning microscope.
Notably for optical record carriers, there is an increasing need of a scanning device with which increasingly smaller details can still be read distinctly, because one wants to store an increasing quantity of information on the record carrier. The quantity of information which can be stored on an optical record carrier depends, inter alia, on the size of the scanning spot which is formed by the scanning device on the information plane of the record carrier. As the scanning spot is smaller, the information density may be larger. The size of the scanning spot may be reduced by increasing the numerical aperture (NA) of the beam with which the scanning spot is formed.
An attractive possibility of increasing the NA without the costs for the objective system becoming too high, and the image field becoming too small, and without causing the problems due to the dispersion of the material from which the lens is formed, is to arrange a planoconvex lens between the actual objective lens and the record carrier. This planoconvex lens, also referred to as solid immersion lens or slider lens, may be situated at a very small distance from the record carrier, but also at a slightly larger distance, for example, 300 .mu.m. The converging function of the objective system is then distributed across he actual objective lens and the planoconvex lens. An advantage of the use of the planoconvex lens is that this lens introduces hardly any aberrations in the radiation beam.
A scanning device provided with such a planoconvex lens is known from EP-A 0 727 777. This device includes an optical scanning head with an objective lens and a planoconvex lens which converge a radiation beam to a numerical aperture of 0.84 for scanning a record carrier. The planoconvex lens may be arranged in a slider which is in sliding contact with the record carrier or floats on an aircushion.
The objective system is sensitive to changes of the refractive index or the temperature. The tolerances for these changes may be alleviated by using separate drivers (actuators) for the objective lens and the planoconvex lens, so that these elements cannot only be moved with respect to the record carrier, but also with respect to each other. The separate drivers complicate the scanning device, render it more sensitive to disturbances, and make it more expensive, which are great disadvantages, particularly for consumer uses. It is therefore preferred, to use a rigid objective system, i.e. a system in which the objective lens and the planoconvex lens are fixed with respect to each other. The elements of the objective system are preferably glass-replica lenses or completely synthetic material lenses. A replica lens is a lens consisting of a basic shape of glass on one or two refracting surface(s) on which a layer of transparent synthetic material is provided. The surface of the synthetic material layer may easily be given an aspherical shape by means of a mold and a replica technique. The glass basic shape with the layer of synthetic material, which is in a sufficiently weak state, is pressed against a mold surface which is the negative of the desired shape and the material is subsequently cured. Also a lens element which consists completely of a synthetic material can easily be given one or two aspherical surface(s), for example, by means of molding. Those skilled in the art are hereby directed to U.S. Pat. No. 4,753,524. The above citations are hereby incorporated herein in whole by reference.